The present invention relates to a radar system having at least one transmitter and one receiver, the at least one antenna of which, for the transmitting and receiving of radar pulses, being arranged at the end of a rotating arm. The radar system has a device for the demodulating and intermediate storage of the reception signals, and devices for forming and storing reference functions as a function of the illumination geometry of the radar system of the range intervals to be measured, of the rotating-angle areas, of the transmitting pulses as well as of the height of the rotating antenna above ground. The system also has a correlator for correlating the reception signals with the reference functions, as well as a display device for displaying the correlation results.
A radar system of this type having a synthetic aperture on the basis of rotating antennae (ROSAR) is known from the journal ntz-Archiv, Volume 9, 1987, Issue 1, Pages 13-23. In that system, the antennae are arranged at the ends of a rotating supporting arm, for example, at the blade tips of helicopter rotors. The radar method using a synthetic aperture is based on the evaluation of Doppler information for each individual point on the ground inside the real antenna lobe and therefore requires a relative movement of the antenna and a pulsed coherent radar system. The relative movement of the antenna, in the case of a ROSAR-system, is generated by the rotating movement of the antennae with respect to a target to be detected. The reception signal is crosscorrelated pulse by pulse by means of a set of reference functions derived from the geometry of illumination, in which case a reference function exists for every distance element or every discrimination cell on the ground. This computer-intensive processing represents an adaptive filter which leads to a higher cross-discrimination than the cross-discrimination that would correspond to the real antenna lobe. In this manner, the discrimination by a synthetic aperture can be increased by one order in comparison to the discrimination by means of a real aperture.
When antennae are arranged at the blade tips of a helicopter rotor, the rotational speed at the tips at approximately 200 m/sec is significantly higher than the cruising speed of the helicopter above ground. The Doppler information, which is generated on the basis of the rotating speed of the tips, is therefore significantly higher than that generated on the basis of the translational movement of the helicopter so that this latter Doppler shift generally does not have to be taken into account. As a result of the antennae mounted in the rotor blade tips, an annulus is illuminated because of their oblique viewing direction downwards, the illuminated strip width being a function of the angle of beam in the elevation, the depression angle, i.e., the angle between the viewing direction of the antenna and the horizontal level, and the height of the antenna above ground. Echoes originating from the illuminated strip, because of their Doppler modulation, are compressed to a higher azimuth discrimination, in which case, for each range point on the ground, the Doppler time function is known as the pertaining reference function. This reference function can be calculated for every plotted point on the ground in a deterministic manner. The whole radar system therefore represents an adaptive filter in which a focussed processing is required.
If, by means of this type of a ROSAR-system, an acceptable discrimination is to be achieved within the range of one meter, a large number of correlation calculations must take place within a short period of time, i.e., an enormous data flow must be processed. A processor which might be able to process this data flow would have to have a special processor structure that is adapted to the ROSAR-principle and could not be provided in the addressed article.
From the German Patent document DE-PS 28 35 932, a ROSAR-system is known in which the antennae are arranged in the blade tips of a helicopter rotor. This system is used as a cartography radar system. In order to simplify the processor structure and to permit conventional signal processing, some limitations are, however, required in that system. Essentially only stationary targets and no moving targets are to be detected. As a result, the correlation calculations may be combined over several periods of rotor rotation. However, for this reason, this ROSAR-system cannot be called a real-time radar system and can therefore not be used, for example, for navigation, and definitely not for target tracking. In addition, in the case of this known concept, the flight altitude of the helicopter above ground is not taken into account because it is constantly assumed to be very large in comparison to the range to the targets. In addition, the described system does not account for the distortion of the illuminated areas by the rotating antennae, i.e., the curvature of the individual discrimination cells. This curvature is called a range curvature.
From U.S. Pat. No. 4,638,315, a radar system having a synthetic aperture is known in which the radar transmitter is fixed, for example, installed in the helicopter cabin, while the reception antenna rotates and is arranged, for example, at the rotor blade tip. The radar transmitter illuminates the area to be monitored while the reflected signals are received by the rotating antenna. The resulting Doppler shift is compensated by the generating of a pilot signal. By a mixing of frequencies, a range signal can then be received. In the true sense of the meaning, this radar system is not a ROSAR-system because no reference functions and no crosscorrelation are formed.
It is an object of the present invention to provide a processor structure, particularly for the formation of the reference functions, by which the discrimination of the ROSAR-system is increased and distortions of the calculated data are avoided as much as possible.